The inductively coupled plasma (ICP) has been a standard excitation/ionization source for the analytical techniques of optical emission spectroscopy (OES) and elemental mass spectrometry (MS) for many years. Unfortunately, the general trend toward more compact and portable spectrochemical sources has been slower to reach elemental (atomic) spectrometry than other fields, largely because the ICP source has remained the workhorse of the field. In addition to their size, ICP sources typically consume large amounts of gas and sample solution.
The liquid sampling, atmospheric pressure, glow discharge (LS-APGD) is a low power, small footprint source that has been used in analytical techniques such as optical emission spectroscopy and mass spectrometry (see, e.g., U.S. Pat. Nos. 6,852,969, 6,750,449, 5,325,021, 5,086,226, and 5,006,706, all of which are incorporated herein by reference). The microplasma of these methods operates at power densities of about 10 W/mm3, much higher than the typically-cited value of about 0.1 W/mm3 for ICP. LS-APGD was originally developed for applications in metal speciation, being operable at low solution flow rates (<400 μL/min) and employing an electrolytic solution (5% acid/salt) as the mobile phase. In the direct solution analysis mode, heat is generated as current flows across the air/liquid interface and causes evaporation, eventually culminating in excitation of the analyte species passing through the microplasma. For quantitative analysis, detection limits for aqueous samples are at the single nanogram level (using relatively simple optical spectrometer systems). The microplasma environment (e.g., kinetic and excitation temperatures) is more in line with combustion flames than other atmospheric pressure plasmas (e.g. ICPs). The robustness of the microplasma with regard to changes in solution matrices is similar to ICP sources. The use of LS-APGD has positive attributes in terms of design simplicity, small footprint, low operating powers, and very low liquid flow rates resulting in no liquid waste. Building on the development of the low power microplasma, the LS-APGD source has brought compactness and low gas and sample consumption to the elemental analysis of flowing samples.
Laser ablation has become a prominent technology for sample introduction in direct chemical analysis of solids, particularly in the case of microanalysis. For analytical purposes, laser ablation has been commonly based on two primary measurement modalities: laser-induced breakdown spectroscopy (LIBS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). LIBS-based analyses provide unique advantages that include rapid in situ simultaneous multi-element analysis. LA-ICP-MS-based analyses provide enhanced sensitivity and the opportunity of isotopic analysis over LIBS, but require more expensive laboratory-based instrumentation. One of the main advantages of LIBS is in situ analysis—no secondary excitation source. This capability is available because the sampling, vaporization, and excitation processes are complete in one step, while in the case of LA-ICP-MS, the sampling and vaporization/excitation/ionization processes are completed in separate (in time and space) steps; consequently they may be individually optimized.
In both technologies, a focused laser beam converts a small portion of a solid sample into an aerosol. For LA-ICP-MS, the ideal analyte of the aerosol is comprised of small, uniform-sized particles that can be entrained and transported efficiently to the ICP. For LIBS, the ideal analyte of the aerosol is vapor that is excited to atomic and ionic optical emission, as particulates do not contribute to the measurement. Fundamentally, both aerosol forms can be produced, as well as aerosols carrying liquid phase analyte, with the composition established by the experimental (ablation) parameters.
For LIBS, the most successful way (to date) to improve sensitivity has been by increasing excitation efficiency in the laser-induced plasma with the use of a second laser pulse (double pulse, DP) delayed from the sampling pulse. The enhancement in spectral line emission intensity using DP excitation depends on several parameters, including inter-pulse delay time, plasma density, laser wavelength, and line excitation energy. The enhancement is proposed to be due to higher plasma temperature and/or larger and longer plasma duration, as well as increased ablated mass (particularly in the case of collinear configurations). Extension to the use of the ICP source to enhance the analyte excitation/ionization is well characterized and implemented across a diverse range of applications. However, there still exist questions as to the practicality of using a 1-2.5 kW rf plasma, and having a sampling volume of about 1 cm3 to analyze sample mass on the order of nanograms and below.
Overall, laser ablation is well established as an excellent method for direct solid sampling and introduction of the sample into conventional spectroscopic sources such as the ICP. The method allows real-time analysis without sample preparation and requires a significantly lower quantity of mass than conventional sample dissolution procedures. Research over the years has addressed laser wavelength and pulse duration as critical parameters defining accuracy, sensitivity and precision. The femtosecond pulsed ablation process has been shown to produce a narrow, nanometer-sized, particle distribution that is ideal for consumption in the ICP. However, as the spatial resolution is improved and the quantity of sampled mass is significantly reduced, the conventional ICP torch becomes a rather large diluting source that is not necessary for the digestion of femtograms or less material.
What is needed in the art are secondary sources for vaporization, excitation, and/or ionization of an analyte that are of physical dimensions that are complementary to the small sample sizes such as are present in laser ablation samples. It would be highly beneficial if such sources were available at much lower operational costs, smaller footprint, lower energy consumption, and with a practical cost to benefit ratio.